I was a bit disappointed by not seeing more Asian characters on the show... but I thought that several of the main cast, at least, looked like they could be of mixed heritage (the Tams, as you said... but also Inara and Zoe), which it seems like would be more common than finding people strictly of one race or another, that far into a future where mankind has been transplanted to another system.

I always thought it was rather brave, or perhaps foolhardy, of Joss to pick Chinese as one of the two major languages... considering exactly how hard it is to get right!